1. Technical Field
The present invention generally relates to Micro-Electro-Mechanical Systems (MEMS). More particularly, the present invention relates to a MEMS structure including an anodically bonded silicon-on-insulator substrate.
2. Background Information
There has been a wide range of fabrication methods developed to fabricate microsensors and microactuators. These devices can be used for sensing, measurement and displacement in the range of micrometers or sub-micrometers. The sensitivity of such kind of devices can be high because of their extremely small dimensions. But their fabrication is quite a challenge due to the strict requirement of process reproducibility and wafer uniformity. Conventional fabrication methods have inherent non-uniformity across full wafer size in both thin film deposition and etching.
It is a common practice to form a heavily B-doped silicon layer and use this as an etch stop to construct a thin silicon membrane. However, there are three key disadvantages associated with the use of a heavily boron-doped silicon layer. One is that the boron doping process requires a long drive-in time. If the silicon membrane thickness is large, it becomes practically impossible to fabricate a freely suspended membrane. The second disadvantage is that the etch chemical is very toxic to allow an acceptable etch stop on boron-doped silicon. Heavy silicon doping also introduces stress into the fabricated silicon membrane and performance degradation can result. A non-uniform boron doping profile (boron concentration decays as it goes deeper into the wafer) can also yield a membrane with a poorly defined interface.
Thus, a need exists for a way to make a well-defined thin silicon membrane for a sensor.